In many applications, it is desirable to provide an illuminating source with a control system for automatically varying the intensity of the illumination provided by the source in response to variations in the ambient light level. For example, a digital or analog clock generally contains a light source in close proximity to the clock face or digital readout to facilitate reading of the time in a darkened room. The light source may be positioned in front of the face to provide for reading of the clock or digital display in reflected light therefrom. The light source may be behind a transparent or translucent digital display or clock face to allow the observer to read the time by the light transmitted through the digital display or clock face. In some clocks and clock radios, the time may be displayed on a digital readout tube, selected elements of which are made to glow corresponding to the different numbers.
In many such applications, no provision is made for decreasing the intensity of the light emitted by the illuminating source to light the clock. However, the light continuously emitted from the clock face or digital display may prove disturbing to, for example, the observer who has so situated the clock so that it may be read from a bed. As a result, the light from the clock may prove disturbing to the observer who may be attempting to sleep with the light from the clock shining in his face. In some cases, the light sources provided for such clocks have manual adjustments for decreasing the intensity of the emitted light.
A major disadvantage with such manual adjustments, of course, is that if the observer adjusts the light intensity so that it is least distracting when the room is dark, he may find it difficult or impossible to read the clock when the ambient light level in the room is high. Conversely, if he adjusts the clock for high light output to enable him to read the clock during periods of increased room light, the clock light output is excessively bright and distracting when he is trying to sleep. Thus, the clock light output requires constant adjustment for ambient light for the comfort and reading ease of the observer.